Vietnamese protesters demonstrate in Ho Chi Minh city on Sunday against a proposal to grant companies lengthy land leases.
Photograph: Kao Nguyen/AFP/Getty Images

The authorities in Vietnam are formally investigating an American man of Vietnamese descent accused of involvement in anti-government protests.

William Anh Nguyen, born in 1985, was one of thousands of people who protested in Ho Chi Minh city on Sunday against a draft law to develop economic zones under which land leases of up to 99 years will be offered to investors. The American from Houston, Texas, is currently studying in Singapore.

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Protesters fear the leases might be snapped up by investors from powerful neighbour China, with which Vietnam has a rocky history.

Nguyen was “gathering and causing trouble” in Ho Chi Minh city on Sunday and was filmed on camera urging others to climb over barricades, the state-run Vietnam News Agency reported. Video footage and witness accounts of a subsequent police crackdown show Nguyen dragged through the streets by plain-clothed police, bleeding from a head injury.

State media reported on the weekend that Nguyen had been prosecuted by the Vietnamese police for disrupting the peace, but the US embassy has confirmed he is still under investigation and no formal charges had been brought.

The relationship between China and Vietnam has long been strained, but with the growth of Chinese influence across south-east Asia – mainly through infrastructure projects as part of their Belt and Road Initiative – and China’s increasing encroachment across the South China Sea, some Vietnamese have become fearful of their neighbour.

Nguyen was one of more than 100 arrested in the demonstrations, which also took place in Hanoi and central Vietnam’s Khanh Hoa and Binh Thuan provinces, where protesters faced tear gas and water cannon when they tried to storm the office of the People’s Committee.

A US state department official said it had raised concerns and engaged with Vietnamese authorities on the matter and that the Vietnamese government permitted consular access to Nguyen on Friday.

The official said they were deeply concerned by reports that Nguyen was injured on Sunday at the time he was taken into custody by Vietnamese authorities and that they would continue to push for continued and regular access to Nguyen.

Vietnam’s foreign ministry spokeswoman Le Thi Thu Hang denied any use of force against Nguyen during a news conference on Thursday.

Ho Chi Minh police said on their website on Friday that they were dealing with 310 people over the protests and that seven of them were criminal cases. It was unclear if Nguyen was one of the seven.

They said initial investigations showed there were signs of political opponents and reactionary groups having incited people to protest and destabilise the security and political situation.

Vietnam is one of several countries in the region that have claims in the South China Sea, through which an estimated $5tn in trade passes each year.

Reuters contributed to this report

An earlier version of this article stated that Nguyen had been formally charged by the Vietnamese police. He is still under investigation and has not been prosecuted.